[
https://issues.apache.org/jira/browse/HIVE-5093?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13744063#comment-13744063
]
Gopal V commented on HIVE-5093:
-------------------------------
[~appodictic]: HIVE-3562 is actually doing this in the map-side aggregation
with a java.util.SortedMap.
I wrote the combiner version for memory efficiency, which is relatively simple
as well.
> Use a combiner for LIMIT with GROUP BY and ORDER BY operators
> -------------------------------------------------------------
>
> Key: HIVE-5093
> URL: https://issues.apache.org/jira/browse/HIVE-5093
> Project: Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 0.12.0
> Reporter: Gopal V
> Assignee: Gopal V
> Attachments: HIVE-5093-WIP-01.patch
>
>
> Operator trees of the following structure can have a memory friendly combiner
> put in place after the sort-phase
> "GBY-LIM" and "OBY-LIM"
> This will cut down on I/O when spilling to disk and particularly during the
> merge phase of the reducer.
> There are two possible combiners - LimitNKeysCombiner and
> LimitNValuesCombiner.
> The first one would be ideal for the GROUP-BY case, while the latter would
> more useful for the ORDER-BY case.
> The combiners are still relevant even if there are 1:1 forward operators on
> the reducer side and for small data items, the MR base layer does not run the
> combiners at all.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira
